A conventional load detection sensor includes a pressure sensitive member and an electrically insulation member. The sensor is disclosed in JP-A-2005-172793, which corresponds to U.S. Pat. No. 7,059,203. The pressure sensitive member has electric characteristics, which are changeable in accordance with stress applied to the sensor. The electrically insulation member is integrally formed on both sides of the pressure sensitive member. The pressure sensitive member is made of glass matrix including conductive particles dispersed in the matrix. The conductive particles are made of, for example, RuO2. The thickness of the pressure sensitive member is in a range between 1 μm and 200 μm. A pair of electrodes is disposed on the pressure sensitive member.
A preliminary load may be applied to the load detection sensor so that not only a load along with a pressing direction but also a load along with a tensile direction is measured. This is disclosed in JP-A-2005-249598 corresponding to U.S. Pat. No. 6,997,060 and JP-A-H11-37860.
However, although JP-A-2005-249598 and JP-A-H11-37860 teaches effects of the preliminary load application, a specific method for applying the load preliminary is not described in JP-A-2005-249598 and JP-A-H11-37860.
The magnitude of the preliminary load application has an appropriate range, and it is necessary to control the magnitude of the preliminary load application according to dimensions of an individual part. The load detection sensor does not have sufficient durability against a rotation torque, compared with compression. When the method for controlling the magnitude of the preliminary load application is a screw method disclosed in JP-A-2005-249598 and JP-A-H11-37860, both of the preliminary load and a rotation torque are applied to the load detection sensor.
The load detection sensor has high rigidity in the compression direction. The screw method does not provide to apply the preliminary load sufficiently. When a screw is loosened, the preliminary load is much reduced. Further, the magnitude of the preliminary load is varied according to accuracy of a surface on which the screw contacts. However, it is difficult to form the surface having sufficient accuracy for securing the sufficient preliminary load. The preliminary load is applied to a small area of the surface, so that durability of the surface is reduced.